1. Field of the Invention
The invention relates to a method for operating a high-temperature fuel cell installation, and a high-temperature fuel cell installation.
Fuels that contain hydrocarbons such as natural gas, heating oil, naphtha and biogas are used for operating high-temperature fuel cell installations. As a rule, these fuels have to be processed in a suitable manner. That is to say, the gas must be reformed before it is fed to the high-temperature fuel cells. The individual high-temperature fuel cells are combined to form high-temperature fuel cell modules.
After being moistened, the fuels containing hydrocarbons pass through a reformation process during which CO, H.sub.2, CO.sub.2 and H.sub.2 O are produced as gaseous reformation products. The gaseous reformation products, which are also called reformate, now form a suitable combustion gas for the operation of the high-temperature fuel cell module.
The process of reformation may in this case be carried out externally or internally, that is to say inside or outside of the high-temperature fuel cell module with or without using the enthalpy of an anode exhaust gas from the high-temperature fuel cell module.
Internal reformation is known, for example, from the report "Verfahrenstechnik der Hochtemperaturbrennstoffzelle" [Process Engineering For High-Temperature Fuel Cells] by E. Riensche, VDI Reports 1174 (1995), pages 63 to 78, in which high-enthalpy waste heat that is produced during the electrochemical combustion in the high-temperature fuel cell module is used for internal reformation of the combustion gas. If the reformation is carried out in the high-temperature fuel cell module but outside an anode part, the process is called indirect internal reformation. Reformation in the anode part is correspondingly called direct internal reformation.
The external reformers which are known for performing the external reformation process, in particular from the report "Erzeugung und Konditionierung von Gasen fur den Einsatz in Brennstoffzellen" [Production And Conditioning Of Gases For Use In Fuel Cells] by K. H. van Heek, VDI Reports 1174 (1995), pages 97 to 116, are configured and constructed such that the amount of combustion gas that is reformed corresponds exactly to the amount required for conversion in the high-temperature fuel cell module for the electrochemical combustion. This configuration applies equally to direct and indirect internal reformation.
Further fuel cell installations are known to include at least one fuel cell stack composed of individual cells (a modular structure) that have an anode part and a cathode part. The combustion gas that is required for the electrochemical reaction being produced by the reformation process in these installations are known from Non-Prosecuted German Patent Application Nos. 43 30 623 A1 and 40 32 652 A1 and from European Patent Application 0 430 017 A2.
The high-temperature fuel cell installations that are known from the prior art are configured for optimally high combustion gas utilization in the electrochemical reaction. The reformed combustion gas is thus used exclusively for the purpose of utilization within the high-temperature fuel cell installation.
As a result of this configuration, a combustion gas concentration gradient exists over the entire active area for the electrochemical reaction within the high-temperature fuel cell module. This results in a weakening of the combustion gas over the active area and may amount to up to 80-90% at the output from the high-temperature fuel cell module. This weakening leads to diffusion constriction within the electrolyte in the high-temperature fuel cell module, which, in turn, leads to considerable reductions in the power density of the high-temperature fuel cell module.